A newly described hereditary cataract in the deer mouse, Peromyscus maniculatus, has been under intensive in vivo biomicroscopic and histologic study for the past year. The cataract is unique in that it resembles the human senile cataract in developing late in the animal's life. It is also unique in that the cataract will develop only in Peromyscus born with the third and fourth toes of their hind feed fused. The life history of this cataract has been traced, from faulty development of the choroidal fissure with colobomas in some animals of optic nerve and choroid, rarely of iris and lens, and a retinal dysplasia-like growth of fibrous and/or glial tissue in the choroidal fissure. Some of the deer mice progress to a condition resembling persistent hyperplastic primary vitreous in humans, when the normal hyaloid vessels and tunica vasculosa lentis do not atrophy. The next step in the disorder is backward migration of lens epithelium from the lens bow over the normal acellular posterior lens. Then, cataracts develop, first in the posterior subcapsular area, then the nuclear, equatorial cortical and anterior cortical area of the lens. The model of human senile cataract provides a unique opportunity for biochemical investigation of the normal lens, the earliest stages in cataract and later changes. Ultramicroscopic changes will be studied by electron microscopy. Using this model, we hope to better understand the pathogenesis of human cataract. BIBLIOGRAPHIC REFERENCES: Burns, R.P., and L. Feeney. Hereditary cataracts in deer mice (Peromyscus maniculatus). Am. J. Ophthalmol. 80: 370-378, 1975.